The proliferation of mobile devices such as smart phones, netbooks and tablet computers has led to a growing need to obtain accurate location information of such devices. Wireless positioning may involve obtaining wireless signals and processing the signals into a location estimate. The typical information used for positioning, such as Global Positioning System (GPS) signals, may be processed to find the position of a wireless device. For example, triangulation may be used where multiple range or angle measurements from known positions are used to calculate the position of a device.
One of the sources of errors in wireless positioning is multipath propagation. Multipath propagation occurs when a signal takes different paths when propagating from a source to a destination receiver. While the signal is traveling, objects get in the way and may cause the signal to bounce in different directions before getting to the receiver. As a result, some of the signals may be delayed and travel along longer paths to the receiver. In other instances, there may be no direct line of sight because an object is completely blocking the signal and any received signals occur only due to multipath propagation. These effects may cause errors in GPS data. The computed position of the device using common techniques such as triangulation may accordingly be incorrect.
The location-aware systems may differ in terms of accuracy, coverage, cost of installation, and maintenance of the systems. The GPS systems may use satellite signals and work in outdoor environments. However, they require direct line of sight and do not work well in an indoor environment. Cell tower triangulation is another method that uses signals from cellular towers to locate a wireless user. This method may also be limited in accuracy and reliability because of the coarse number of cell towers from a particular service provider that a mobile user can communicate with, as well as multipath issues.
Systems have been developed in the past that use the strength of wireless access point beacon signals in an outdoor environment to calculate the position of a mobile user. One technique is to create a database of wireless beacons and use that information together with the amplitude of beacons signals received by a mobile device to compute the location of the mobile device. Other techniques use radio frequency (RF) wireless signal strength information and triangulation to locate objects in an indoor environment. However, these methods provide poor indoor positioning accuracy because RF signal amplitude is greatly affected by metal objects, reflective surfaces, multipath, dead-spots, noise and interference.
Other methods use time of arrival information of cellular RF signals and cell tower triangulation to determine a coarse radio-based position and then use that information to assist a GPS system to determine the GPS-based position. Cell tower triangulation may be limited in accuracy and reliability because of the coarse number of cell towers and multipath issues. Also, these methods may require the presence of a GPS signal and a GPS time reference for measuring time of arrival of cellular RF signals. This approach may not work in indoor situations where GPS signals are weak or not present. These methods also require time synchronization and prior knowledge of the position of cellular base-stations. Other methods have also used cellular tower triangulation to determine a coarse pre-fix position and use that to assist a GPS system to determine the GPS-based position.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.